Solar sails (also known aslightsails,light sails, andphoton sails) are a method ofspacecraft propulsion usingradiation pressure exerted bysunlight on large surfaces. A number of spaceflight missions to test solar propulsion and navigation have been proposed since the 1980s. The first spacecraft to make use of the technology wasIKAROS, launched in 2010.[1]
A useful analogy to solar sailing may be asailing boat; the light exerting aforce on the large surface is akin to a sail being blown by the wind. High-energylaser beams could be used as an alternative light source to exert much greater force than would be possible using sunlight, a concept known as beam sailing. Solar sail craft offer the possibility of low-cost operations combined with high speeds (relative tochemical rockets) and long operating lifetimes. Since they have few moving parts and use no propellant, they can potentially be used numerous times for the delivery of payloads.
Solar sails use a phenomenon that has a proven, measured effect onastrodynamics. Solar pressure affects all spacecraft, whether ininterplanetary space or in orbit around a planet or small body. A typical spacecraft going to Mars, for example, will be displaced thousands of kilometers by solar pressure, so the effects must be accounted for in trajectory planning, which has been done since the time of the earliest interplanetary spacecraft of the 1960s. Solar pressure also affects theorientation of a spacecraft, a factor that must be included inspacecraft design.[2]
The total force exerted on an 800 by 800 metres (2,600 by 2,600 ft) solar sail, for example, is about 5 N (1.1 lbf) at Earth's distance from the Sun,[3] making it a low-thrustpropulsion system, similar to spacecraft propelled byelectric engines, but as it uses no propellant, that force is exerted almost constantly and the collective effect over time is great enough to be considered a potential manner of propelling spacecraft.
Johannes Kepler observed thatcomet tails point away from theSun and suggested that the Sun caused the effect. In a letter to Galileo in 1610, he wrote, "Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void."[4] He might have had the comet tail phenomenon in mind when he wrote those words, although his publications on comet tails came several years later.[5]
James Clerk Maxwell, in 1861–1864, published his theory ofelectromagnetic fields and radiation, which shows that light hasmomentum and thus can exert pressure on objects.Maxwell's equations provide the theoretical foundation for sailing with light pressure. So by 1864, the physics community and beyond knewsunlight carried momentum that would exert a pressure on objects.
Jules Verne, inFrom the Earth to the Moon,[6] published in 1865, wrote "there will some day appear velocities far greater than these [of the planets and the projectile], of which light or electricity will probably be the mechanical agent ... we shall one day travel to the moon, the planets, and the stars."[7] This is possibly the first published recognition that light could move ships through space.
Pyotr Lebedev was first to successfully demonstrate light pressure, which he did in 1899 with a torsional balance;[8] Ernest Nichols and Gordon Hull conducted a similar independent experiment in 1901 using aNichols radiometer.[9]
Svante Arrhenius predicted in 1908 the possibility of solar radiation pressure distributing life spores across interstellar distances, providing one means to explain the concept ofpanspermia. He was apparently the first scientist to state that light could move objects between stars.[10]
Konstantin Tsiolkovsky first proposed using the pressure of sunlight to propel spacecraft through space and suggested, "using tremendous mirrors of very thin sheets to utilize the pressure of sunlight to attain cosmic velocities".[11]
Friedrich Zander (Tsander) published a technical paper in 1925 that included technical analysis of solar sailing. Zander wrote of "applying small forces" using "light pressure or transmission of light energy to distances by means of very thin mirrors".[12]
JBS Haldane speculated in 1927 about the invention of tubular spaceships that would take humanity to space and how "wings of metallic foil of a square kilometre or more in area are spread out to catch the Sun's radiation pressure".[13]
J. D. Bernal wrote in 1929, "A form of space sailing might be developed which used the repulsive effect of the Sun's rays instead of wind. A space vessel spreading its large, metallic wings, acres in extent, to the full, might be blown to the limit of Neptune's orbit. Then,to increase its speed, it would tack, close-hauled, down the gravitational field, spreading full sail again as it rushed past the Sun."[14]
Arthur C. Clarke wroteSunjammer, ascience fictionshort story originally published in the March 1964 issue ofBoys' Life[15] depicting a yacht race between solar sail spacecraft.
Carl Sagan, in the 1970s, popularized the idea of sailing on light using a giant structure which would reflectphotons in one direction, creating momentum. He brought up his ideas in college lectures, books, and television shows. He was fixated on quickly launching this spacecraft in time to perform a rendezvous withHalley's Comet. Unfortunately, the mission didn't take place in time and he would never live to finally see it through.[16]
The first formal technology and design effort for a solar sail began in 1976 atJet Propulsion Laboratory for a proposed mission to rendezvous withHalley's Comet.[3]
Most solar sails are based onreflection.[17] The surface of the sail is highly reflective, like amirror, and light reflecting off of the surface imparts a force.
In 2018,diffraction was proposed as a different solar sail propulsion mechanism, which is claimed to have several advantages.[18][19]
Pekka Janhunen fromFMI has proposed a type of solar sail called theelectric solar wind sail.[20] Mechanically it has little in common with the traditional solar sail design. The sails are replaced with straightened conducting tethers (wires) placedradially around the host ship. The wires are electrically charged to create anelectric field around the wires. The electric field extends a few tens of metres into the plasma of the surrounding solar wind. The solar electrons are reflected by the electric field (like the photons on a traditional solar sail). The radius of the sail is from the electric field rather than the actual wire itself, making the sail lighter. The craft can also be steered by regulating the electric charge of the wires. A practical electric sail would have 50–100 straightened wires with a length of about 20 km each.[21]
Electric solar wind sails can adjust their electrostatic fields and sail attitudes.
Amagnetic sail would also employ the solar wind. However, the magnetic field deflects the electrically charged particles in the wind. It uses wire loops, and runs a static current through them instead of applying a static voltage.[22]
All these designs maneuver, though the mechanisms are different.
Magnetic sails bend the path of the charged protons that are in thesolar wind. By changing the sails' attitudes, and the size of the magnetic fields, they can change the amount and direction of the thrust.
The force imparted to a solar sail arises from the momentum of photons. The momentum of aphoton or an entire flux is given byEinstein's relation:[23][24]
where p is the momentum, E is the energy (of the photon or flux), and c is thespeed of light. Specifically, the momentum of a photon depends on its wavelengthp =h/λ
Solar radiation pressure can be related to the irradiance (solar constant) value of 1361 W/m2 at 1 AU (Earth-Sun distance), as revised in 2011:[25]
An ideal sail is flat and has 100%specular reflection. An actual sail will have an overall efficiency of about 90%, about 8.17 μN/m2,[24] due to curvature (billow), wrinkles, absorbance, re-radiation from front and back, non-specular effects, and other factors.
The force on a sail and the actual acceleration of the craft vary by the inverse square of distance from the Sun (unless extremely close to the Sun[26]), and by the square of the cosine of the angle between the sail force vector and the radial from the Sun, so
where R is distance from the Sun in AU. An actual square sail can be modelled as:
Note that the force and acceleration approach zero generally around θ = 60° rather than 90° as one might expect with an ideal sail.[27]
If some of the energy is absorbed, the absorbed energy will heat the sail, which re-radiates that energy from the front and rear surfaces, depending on theemissivity of those two surfaces.
Solar wind, the flux of charged particles blown out from the Sun, exerts a nominal dynamic pressure of about 3 to 4nPa, three orders of magnitude less than solar radiation pressure on a reflective sail.[28]
Sail loading (areal density) is an important parameter, which is the total mass divided by the sail area, expressed in g/m2. It is represented by the Greek letter σ (sigma).
A sail craft has a characteristic acceleration, ac, which it would experience at 1 AU when facing the Sun. Note this value accounts for both the incident and reflected momentums. Using the value from above of 9.08 μN per square metre of radiation pressure at 1 AU, ac is related to areal density by:
Assuming 90% efficiency, ac = 8.17 / σ mm/s2
The lightness number, λ, is the dimensionless ratio of maximum vehicle acceleration divided by the Sun's local gravity. Using the values at 1 AU:
The lightness number is also independent of distance from the Sun because both gravity and light pressure fall off as the inverse square of the distance from the Sun. Therefore, this number defines the types of orbit maneuvers that are possible for a given vessel.
The table presents some example values. Payloads are not included. The first two are from the detailed design effort at JPL in the 1970s. The third, the lattice sailer, might represent about the best possible performance level.[3] The dimensions for square and lattice sails are edges. The dimension for heliogyro is blade tip to blade tip.
| Type | σ (g/m2) | ac (mm/s2) | λ | Size (km2) |
|---|---|---|---|---|
| Square sail | 5.27 | 1.56 | 0.26 | 0.820 |
| Heliogyro | 6.39 | 1.29 | 0.22 | 15 |
| Lattice sailer | 0.07 | 117 | 20 | 0.840 |
An activeattitude control system (ACS) is essential for a sail craft to achieve and maintain a desired orientation. The required sail orientation changes slowly (often less than 1 degree per day) in interplanetary space, but much more rapidly in a planetary orbit. The ACS must be capable of meeting these orientation requirements. Attitude control is achieved by a relative shift between the craft'scenter of pressure and itscenter of mass. This can be achieved with control vanes, movement of individual sails, movement of a control mass, or altering reflectivity.
Holding a constant attitude requires that the ACS maintain a net torque of zero on the craft. The total force and torque on a sail, or set of sails, is not constant along a trajectory. The force changes with solar distance and sail angle, which changes the billow in the sail and deflects some elements of the supporting structure, resulting in changes in the sail force and torque.
Sail temperature also changes with solar distance and sail angle, which changes sail dimensions. The radiant heat from the sail changes the temperature of the supporting structure. Both factors affect total force and torque.
To hold the desired attitude the ACS must compensate for all of these changes.[29]
In Earth orbit, solar pressure and drag pressure are typically equal at an altitude of about 800 km, which means that a sail craft would have to operate above that altitude. Sail craft must operate in orbits where their turn rates are compatible with the orbits, which is generally a concern only for spinning disk configurations.
Sail operating temperatures are a function of solar distance, sail angle, reflectivity, and front and back emissivities. A sail can be used only where its temperature is kept within its material limits. Generally, a sail can be used rather close to the Sun, around 0.25 AU, or even closer if carefully designed for those conditions.[3]
Potential applications for sail craft range throughout theSolar System, from near the Sun to the comet clouds beyond Neptune. The craft can make outbound voyages to deliver loads or to take up station keeping at the destination. They can be used to haul cargo and possibly also used for human travel.[3]
For trips within the inner Solar System, they can deliver payloads and then return to Earth for subsequent voyages, operating as an interplanetary shuttle. For Mars in particular, the craft could provide economical means of routinely supplying operations on the planet. According to Jerome Wright, "The cost of launching the necessary conventional propellants from Earth are enormous for manned missions. Use of sailing ships could potentially save more than $10 billion in mission costs."[3]
Solar sail craft can approach the Sun to deliver observation payloads or to take up station keeping orbits. They can operate at 0.25 AU or closer. They can reach high orbital inclinations, including polar.
Solar sails can travel to and from all of the inner planets. Trips to Mercury and Venus are for rendezvous and orbit entry for the payload. Trips to Mars could be either for rendezvous or swing-by with release of the payload foraerodynamic braking.[3]
| Sail size m | Mercury Rendezvous | Venus Rendezvous | Mars Rendezvous | Mars Aerobrake | ||||
|---|---|---|---|---|---|---|---|---|
| days | tons | days | tons | days | tons | days | tons | |
| 800 σ = 5 g/m2 w/o cargo | 600 | 9 | 200 | 1 | 400 | 2 | 131 | 2 |
| 900 | 19 | 270 | 5 | 500 | 5 | 200 | 5 | |
| 1200 | 28 | 700 | 9 | 338 | 10 | |||
| 2000 σ = 3 g/m2 w/o cargo | 600 | 66 | 200 | 17 | 400 | 23 | 131 | 20 |
| 900 | 124 | 270 | 36 | 500 | 40 | 200 | 40 | |
| 1200 | 184 | 700 | 66 | 338 | 70 | |||
Minimum transfer times to the outer planets benefit from using an indirect transfer (solar swing-by). However, this method results in high arrival speeds. Slower transfers have lower arrival speeds.
The minimum transfer time to Jupiter forac of 1 mm/s2 with no departure velocity relative to Earth is 2 years when using an indirect transfer (solar swing-by). The arrival speed (V∞) is close to 17 km/s. For Saturn, the minimum trip time is 3.3 years, with an arrival speed of nearly 19 km/s.[3]
| Jupiter | Saturn | Uranus | Neptune | |
|---|---|---|---|---|
| Time, yr | 2.0 | 3.3 | 5.8 | 8.5 |
| Speed, km/s | 17 | 19 | 20 | 20 |
The Sun's innergravitational focus point lies at minimum distance of 550 AU from the Sun, and is the point to which light from distant objects isfocused by gravity as a result of it passing by the Sun. This is thus the distant point to which solar gravity will cause the region of deep space on the other side of the Sun to be focused, thus serving effectively as a very large telescope objective lens.[30][31]
It has been proposed that an inflated sail, made ofberyllium, that starts at 0.05 AU from the Sun would gain an initial acceleration of 36.4 m/s2, and reach a speed of 0.00264c (about 950 km/s) in less than a day. Such proximity to the Sun could prove to be impractical in the near term due to the structural degradation of beryllium at high temperatures, diffusion of hydrogen at high temperatures as well as an electrostatic gradient, generated by the ionization of beryllium from the solar wind, posing a burst risk. A revised perihelion of 0.1 AU would reduce the aforementioned temperature and solar flux exposure.[32]Such a sail would take "Two and a half years to reach theheliopause, six and a half years to reach the Sun’s innergravitational focus, with arrival at the inner Oort Cloud in no more than thirty years."[31] "Such a mission could perform useful astrophysical observations en route, explore gravitational focusing techniques, and image Oort Cloud objects while exploring particles and fields in that region that are of galactic rather than solar origin."
Robert L. Forward has commented that a solar sail could be used to modify the orbit of a satellite about the Earth. In the limit, a sail could be used to "hover" a satellite above one pole of the Earth. Spacecraft fitted with solar sails could also be placed in close orbits such that they are stationary with respect to either the Sun or the Earth, a type of satellite named by Forward a "statite". This is possible because the propulsion provided by the sail offsets the gravitational attraction of the Sun. Such an orbit could be useful for studying the properties of the Sun for long durations.[33] Likewise a solar sail-equipped spacecraft could also remain on station nearly above the polarsolar terminator of a planet such as the Earth by tilting the sail at the appropriate angle needed to counteract the planet's gravity.[33]
In his bookThe Case for Mars,Robert Zubrin points out that the reflected sunlight from a large statite, placed near the polar terminator of the planet Mars, could be focused on one of the Martian polar ice caps to significantly warm the planet's atmosphere. Such a statite could be made from asteroid material.
A group of satellites designed to act as sails has been proposed to measureEarth's Energy Imbalance which is the most fundamental measure of the planet's rate ofglobal warming. On-board state-of-the-artaccelerometers would measure shifts in the pressure differential between incoming solar and outgoingthermal radiation on opposing sides of each satellite. Measurement accuracy has been projected to be better than that achievable with compactradiometric detectors.[34]
TheMESSENGER probe orbitingMercury used light pressure on its solar panels to perform fine trajectory corrections on the way to Mercury.[35] By changing the angle of the solar panels relative to the Sun, the amount of solar radiation pressure was varied to adjust the spacecraft trajectory more delicately than possible with thrusters. Minor errors are greatly amplified bygravity assist maneuvers, so using radiation pressure to make very small corrections saved large amounts of propellant.
In the 1970s,Robert Forward proposed twobeam-powered propulsion schemes using either lasers ormasers to push giant sails to a significant fraction of thespeed of light.[36]
In the science fiction novelRocheworld, Forward described a light sail propelled by super lasers. As the starship neared its destination, the outer portion of the sail would detach. The outer sail would then refocus and reflect the lasers back onto a smaller, inner sail. This would provide braking thrust to stop the ship in the destination star system.
Both methods pose monumental engineering challenges. The lasers would have to operate for years continuously atgigawatt strength. Forward's solution to this requires enormous solar panel arrays to be built at or near the planet Mercury. A planet-sized mirror orFresnel lens would need to be located at several dozenastronomical units from the Sun to keep the lasers focused on the sail. The giant braking sail would have to act as a precision mirror to focus the braking beam onto the inner "deceleration" sail.
A potentially easier approach would be to use a maser to drive a "solar sail" composed of a mesh of wires with the same spacing as the wavelength of the microwaves directed at the sail, since the manipulation of microwave radiation is somewhat easier than the manipulation of visible light. The hypothetical "Starwisp" interstellar probe design[37][38] would use microwaves, rather than visible light, to push it. Masers spread out more rapidly than optical lasers owing to their longer wavelength, and so would not have as great an effective range.
Masers could also be used to power a painted solar sail, a conventional sail coated with a layer of chemicals designed to evaporate when struck by microwave radiation.[39] The momentum generated by thisevaporation could significantly increase thethrust generated by solar sails, as a form of lightweightablative laser propulsion.
To further focus the energy on a distant solar sail, Forward proposed a lens designed as a largezone plate. This would be placed at a location between the laser or maser and the spacecraft.[36]
Another more physically realistic approach would be to use the light from the Sun to accelerate the spacecraft.[40] The ship would first drop into an orbit making a close pass to the Sun, to maximize the solar energy input on the sail, then it would begin to accelerate away from the system using the light from the Sun. Acceleration will drop approximately as the inverse square of the distance from the Sun, and beyond some distance, the ship would no longer receive enough light to accelerate it significantly, but would maintain the final velocity attained. When nearing the target star, the ship could turn its sails toward it and begin to use the outward pressure of the destination star to decelerate. Rockets could augment the solar thrust.
Similar solar sailing launch and capture were suggested fordirected panspermia to expand life in other solar systems. Velocities of 0.05% the speed of light could be obtained by solar sails carrying 10 kg payloads, using thin solar sail vehicles with effective areal densities of 0.1 g/m2 with thin sails of 0.1 μm thickness and sizes on the order of one square kilometer. Alternatively, swarms of 1 mm capsules could be launched on solar sails with radii of 42 cm, each carrying 10,000 capsules of a hundred millionextremophile microorganisms to seedlife in diverse target environments.[41][42]
Theoretical studies suggest relativistic speeds if the solar sail harnesses a supernova.[43]
Small solar sails have been proposed to accelerate the deorbiting of small artificial satellites from Earth orbits. Satellites inlow Earth orbit can use a combination of solar pressure on the sail and increased atmospheric drag to accelerate satellitereentry.[44] A de-orbit sail developed atCranfield University is part of the UK satellite TechDemoSat-1, launched in 2014. The sail deployed at the end of the satellite's five-year useful life in May 2019.[45] The sail's purpose is to bring the satellite out of orbit over a period of about 25 years.[46] In July 2015 British 3UCubeSat calledDeorbitSail was launched into space with the purpose of testing 16 m2 deorbit structure,[47] but eventually it failed to deploy it.[48] A student 2U CubeSat mission calledPW-Sat2, launched in December 2018 and tested a 4 m2 deorbit sail. It successfully deorbited in February 2021.[49] In June 2017, a second British 3UCubeSat calledInflateSail deployed a 10 m2 deorbit sail at an altitude of 500 kilometers (310 mi).[50]In June 2017 the 3U Cubesat URSAMAIOR has been launched inlow Earth orbit to test the deorbiting system ARTICA developed bySpacemind.[51] The device, which occupies only 0.4 U of the cubesat, shall deploy a sail of 2.1 m2 to deorbit the satellite at the end of the operational life.[52]
IKAROS, launched in 2010, was the first practical solar sail vehicle. As of 2015, it was still under thrust, proving the practicality of a solar sail for long-duration missions.[53] It is spin-deployed, with tip-masses in the corners of its square sail. The sail is made of thinpolyimide film, coated with evaporated aluminium. It steers with electrically controlledliquid crystal panels. The sail slowly spins, and these panels turn on and off to control the attitude of the vehicle. When on, they diffuse light, reducing the momentum transfer to that part of the sail. When off, the sail reflects more light, transferring more momentum. In that way, they turn the sail.[54] Thin-filmsolar cells are also integrated into the sail, powering the spacecraft. The design is very reliable, because spin deployment, which is preferable for large sails, simplified the mechanisms to unfold the sail and the LCD panels have no moving parts.
Parachutes have very low mass, but a parachute is not a workable configuration for a solar sail. Analysis shows that a parachute configuration would collapse from the forces exerted by shroud lines, since radiation pressure does not behave like aerodynamic pressure, and would not act to keep the parachute open.[55]
The highest thrust-to-mass designs for ground-assembled deploy-able structures are square sails with the masts andguy lines on the dark side of the sail. Usually there are four masts that spread the corners of the sail, and a mast in the center to holdguy-wires. One of the largest advantages is that there are no hot spots in the rigging from wrinkling or bagging, and the sail protects the structure from the Sun. This form can, therefore, go close to the Sun for maximum thrust. Most designs steer with small moving sails on the ends of the spars.[56]
In the 1970sJPL studied many rotating blade and ring sails for a mission to rendezvous withHalley's Comet. The intention was to stiffen the structures using angular momentum, eliminating the need for struts, and saving mass. In all cases, surprisingly large amounts of tensile strength were needed to cope with dynamic loads. Weaker sails would ripple or oscillate when the sail's attitude changed, and the oscillations would add and cause structural failure. The difference in the thrust-to-mass ratio between practical designs was almost nil, and the static designs were easier to control.[56]
JPL's reference design was called the "heliogyro". It had plastic-film blades deployed from rollers and held out by centrifugal forces as it rotated. The spacecraft's attitude and direction were to be completely controlled by changing the angle of the blades in various ways, similar to the cyclic and collective pitch of ahelicopter. Although the design had no mass advantage over a square sail, it remained attractive because the method of deploying the sail was simpler than a strut-based design.[56] TheCubeSail (UltraSail) is an active project aiming to deploy a heliogyro sail.
Heliogyro design is similar to the blades on a helicopter. The design is faster to manufacture due to lightweight centrifugal stiffening of sails. Also, they are highly efficient in cost and velocity because the blades are lightweight and long. Unlike the square and spinning disk designs, heliogyro is easier to deploy because the blades are compacted on a reel. The blades roll out when they are deploying after the ejection from the spacecraft. As the heliogyro travels through space the system spins around because of the centrifugal acceleration. Finally, payloads for the space flights are placed in the center of gravity to even out the distribution of weight to ensure stable flight.[56]
JPL also investigated "ring sails" (Spinning Disk Sail in the above diagram), panels attached to the edge of a rotating spacecraft. The panels would have slight gaps, about one to five percent of the total area. Lines would connect the edge of one sail to the other. Masses in the middles of these lines would pull the sails taut against the coning caused by the radiation pressure. JPL researchers said that this might be an attractive sail design for large crewed structures. The inner ring, in particular, might be made to have artificial gravity roughly equal to the gravity on the surface of Mars.[56]
A solar sail can serve a dual function as a high-gain antenna.[57] Designs differ, but most modify themetalization pattern to create a holographic monochromatic lens or mirror in the radio frequencies of interest, including visible light.[57]
The most common material in current designs is a thin layer of aluminum coating on a polymer (plastic) sheet, such as aluminized 2 μmKapton film. The polymer provides mechanical support as well as flexibility, while the thin metal layer provides the reflectivity. Such material resists the heat of a pass close to the Sun and still remains reasonably strong. The aluminum reflecting film is on the Sun side. The sails ofCosmos 1 were made ofaluminized PET film (Mylar).
Eric Drexler developed a concept for a sail in which the polymer was removed.[58] He proposed very high thrust-to-mass solar sails, and made prototypes of the sail material. His sail would use panels of thin aluminium film (30 to 100nanometres thick) supported by atensile structure. The sail would rotate and would have to be continually under thrust. He made and handled samples of the film in the laboratory, but the material was too delicate to survive folding, launch, and deployment. The design planned to rely on space-based production of the film panels, joining them to a deployable tension structure. Sails in this class would offer high area per unit mass and hence accelerations up to "fifty times higher" than designs based on deploy-able plastic films.[58]The material developed for the Drexler solar sail was a thin aluminium film with a baseline thickness of 0.1 μm, to be fabricated by vapor deposition in a space-based system. Drexler used a similar process to prepare films on the ground. As anticipated, these films demonstrated adequate strength and robustness for handling in the laboratory and for use in space, but not for folding, launch, and deployment.
Research byGeoffrey Landis in 1998–1999, funded by theNASA Institute for Advanced Concepts, showed that various materials such asalumina for laser lightsails andcarbon fiber for microwave pushed lightsails were superior sail materials to the previously standard aluminium or Kapton films.[59]
In 2000, Energy Science Laboratories developed a newcarbon fiber material that might be useful for solar sails.[60][61] The material is over 200 times thicker than conventional solar sail designs, but it is so porous that it has the same mass. The rigidity and durability of this material could make solar sails that are significantly sturdier than plastic films. The material could self-deploy and should withstand higher temperatures.
There has been some theoretical speculation about usingmolecular manufacturing techniques to create advanced, strong, hyper-light sail material, based onnanotube mesh weaves, where the weave "spaces" are less than half the wavelength of light impinging on the sail. While such materials have so far only been produced in laboratory conditions, and the means for manufacturing such material on an industrial scale are not yet available, such materials could mass less than 0.1 g/m2,[62] making them lighter than any current sail material by a factor of at least 30. For comparison, 5 micrometre thickMylar sail material mass 7 g/m2, aluminized Kapton films have a mass as much as 12 g/m2,[56] and Energy Science Laboratories' new carbon fiber material masses 3 g/m2.[60]
The least dense metal islithium, about 5 times less dense than aluminium. Fresh, unoxidized surfaces are reflective. At a thickness of 20 nm, lithium has an area density of 0.011 g/m2. A high-performance sail could be made of lithium alone at 20 nm (no emission layer). It would have to be fabricated in space and not used to approach the Sun. In the limit, a sail craft might be constructed with a total areal density of around 0.02 g/m2, giving it a lightness number of 67 and ac of about 400 mm/s2.Magnesium andberyllium are also potential materials for high-performance sails. These 3 metals can be alloyed with each other and with aluminium.[3]
Aluminium is the common choice for the reflection layer. It typically has a thickness of at least 20 nm, with a reflectivity of 0.88 to 0.90. Chromium is a good choice for the emission layer on the face away from the Sun. It can readily provide emissivity values of 0.63 to 0.73 for thicknesses from 5 to 20 nm on plastic film. Usable emissivity values are empirical because thin-film effects dominate; bulk emissivity values do not hold up in these cases because material thickness is much thinner than the emitted wavelengths.[63]
Sails are fabricated on Earth on long tables where ribbons are unrolled and joined to create the sails. Sail material needed to have as little weight as possible because it would require the use of the shuttle to carry the craft into orbit. Thus, these sails are packed, launched, and unfurled in space.[64]
In the future, fabrication could take place in orbit inside large frames that support the sail. This would result in lower mass sails and elimination of the risk of deployment failure.
Sailing operations are simplest in interplanetary orbits, where altitude changes are done at low rates. For outward bound trajectories, the sail force vector is oriented forward of the Sun line, which increases orbital energy and angular momentum, resulting in the craft moving farther from the Sun. For inward trajectories, the sail force vector is oriented behind the Sun line, which decreases orbital energy and angular momentum, resulting in the craft moving in toward the Sun. It is worth noting that only the Sun's gravity pulls the craft toward the Sun—there is no analog to a sailboat's tacking to windward. To change orbital inclination, the force vector is turned out of the plane of the velocity vector.
In orbits around planets or other bodies, the sail is oriented so that its force vector has a component along the velocity vector, either in the direction of motion for an outward spiral, or against the direction of motion for an inward spiral.
Trajectory optimizations can often require intervals of reduced or zero thrust. This can be achieved by rolling the craft around the Sun line with the sail set at an appropriate angle to reduce or remove the thrust.[3]
A close solar passage can be used to increase a craft's energy. The increased radiation pressure combines with the efficacy of being deep in the Sun's gravity well to substantially increase the energy for runs to the outer Solar System. The optimal approach to the Sun is done by increasing the orbital eccentricity while keeping the energy level as high as practical. The minimum approach distance is a function of sail angle, thermal properties of the sail and other structure, load effects on structure, and sail optical characteristics (reflectivity and emissivity). A close passage can result in substantial optical degradation. Required turn rates can increase substantially for a close passage. A sail craft arriving at a star can use a close passage to reduce energy, which also applies to a sail craft on a return trip from the outer Solar System.
A lunar swing-by can have important benefits for trajectories leaving from or arriving at Earth. This can reduce trip times, especially in cases where the sail is heavily loaded. A swing-by can also be used to obtain favorable departure or arrival directions relative to Earth.
A planetary swing-by could also be employed similar to what is done with coasting spacecraft, but good alignments might not exist due to the requirements for overall optimization of the trajectory.[65]
.png)
The following table lists some example concepts using beamed laser propulsion as proposed by the physicistRobert L. Forward:[66]
| Mission | Laser Power | Vehicle Mass | Acceleration | Sail Diameter | Maximum Velocity (% of the speed of light) |
|---|---|---|---|---|---|
| 1. Flyby – Alpha Centauri, 40 years | |||||
| outbound stage | 65 GW | 1 t | 0.036 g | 3.6 km | 11% @ 0.17 ly |
| 2. Rendezvous – Alpha Centauri, 41 years | |||||
| outbound stage | 7,200 GW | 785 t | 0.005 g | 100 km | 21% @ 4.29 ly |
| deceleration stage | 26,000 GW | 71 t | 0.2 g | 30 km | 21% @ 4.29 ly |
| 3. Crewed – Epsilon Eridani, 51 years (including 5 years exploring star system) | |||||
| outbound stage | 75,000,000 GW | 78,500 t | 0.3 g | 1000 km | 50% @ 0.4 ly |
| deceleration stage | 21,500,000 GW | 7,850 t | 0.3 g | 320 km | 50% @ 10.4 ly |
| return stage | 710,000 GW | 785 t | 0.3 g | 100 km | 50% @ 10.4 ly |
| deceleration stage | 60,000 GW | 785 t | 0.3 g | 100 km | 50% @ 0.4 ly |
| Name | Travel time (yr) | Distance (ly) | Luminosity (L☉) |
|---|---|---|---|
| Sirius A | 68.90 | 8.58 | 24.20 |
| α Centauri A | 101.25 | 4.36 | 1.52 |
| α Centauri B | 147.58 | 4.36 | 0.50 |
| Procyon A | 154.06 | 11.44 | 6.94 |
| Vega | 167.39 | 25.02 | 50.05 |
| Altair | 176.67 | 16.69 | 10.70 |
| Fomalhaut A | 221.33 | 25.13 | 16.67 |
| Denebola | 325.56 | 35.78 | 14.66 |
| Castor A | 341.35 | 50.98 | 49.85 |
| Epsilon Eridani | 363.35 | 10.50 | 0.50 |
.Ref:[67]
Both theMariner 10 mission, which flew by the planetsMercury andVenus, and theMESSENGER mission to Mercury demonstrated the use of solar pressure as a method ofattitude control in order to conserve attitude-control propellant.
Hayabusa also used solar pressure on its solar paddles as a method of attitude control to compensate for brokenreaction wheels and chemical thruster.
MTSAT-1R (Multi-Functional Transport Satellite)'s solar sail counteracts the torque produced by sunlight pressure on the solar array. The trim tab on the solar array makes small adjustments to the torque balance.
NASA has successfully tested deployment technologies on small scale sails in vacuum chambers.[68]
In 1999, a full-scale deployment of a solar sail was tested on the ground at DLR/ESA in Cologne.[69]
Cosmos 1, a joint private project betweenPlanetary Society,Cosmos Studios andRussian Academy of Science attempted to launch a suborbital prototype vehicle in 2005, which was destroyed due to a rocket failure.
A 15-meter-diameter solar sail (SSP, solar sail sub payload,soraseiru sabupeiro-do) was launched together withASTRO-F on aM-V rocket on February 21, 2006, and made it to orbit. It deployed from the stage, but opened incompletely.[70]
On August 9, 2004, the JapaneseISAS successfully deployed two prototype solar sails from a sounding rocket. A clover-shaped sail was deployed at 122 km altitude and a fan-shaped sail was deployed at 169 km altitude. Both sails used 7.5-micrometer film. The experiment purely tested the deployment mechanisms, not propulsion.[71]
On February 4, 1993, theZnamya 2, a 20-meter wide aluminized-mylar reflector, was successfully deployed from the RussianMir space station. It was the first thin film reflector of such type successfully deployed in space using the mechanism based on centrifugal force.[72] Although the deployment succeeded, propulsion was not demonstrated. A second test in 1999,Znamya 2.5, failed to deploy properly.
On 21 May 2010,Japan Aerospace Exploration Agency (JAXA) launched theworld's firstinterplanetary solar sailspacecraft "IKAROS" (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) to Venus.[73] Using a new solar-photon propulsion method,[74] it was the first true solar sail spacecraft fully propelled by sunlight,[75][76] and was the first spacecraft to succeed in solar sail flight.[77]
JAXA successfully tested IKAROS in 2010. The goal was to deploy and control the sail and, for the first time, to determine the minute orbit perturbations caused by light pressure. Orbit determination was done by the nearbyAKATSUKI probe from which IKAROS detached after both had been brought into a transfer orbit to Venus. The total effect over the six month flight was 100 m/s.[78]
Until 2010, no solar sails had been successfully used in space as primary propulsion systems. On 21 May 2010, the Japan Aerospace Exploration Agency (JAXA) launched the IKAROS spacecraft, which deployed a 200 m2 polyimide experimental solar sail on June 10.[79][80][81] In July, the next phase for the demonstration of acceleration by radiation began. On 9 July 2010, it was verified that IKAROS collected radiation from the Sun and began photon acceleration by the orbit determination of IKAROS by range-and-range-rate (RARR) that is newly calculated in addition to the data of the relativization accelerating speed of IKAROS between IKAROS and the Earth that has been taken since before the Doppler effect was utilized.[82] The data showed that IKAROS appears to have been solar-sailing since 3 June when it deployed the sail.
IKAROS has a diagonal spinning square sail 14×14 m (196 m2) made of a 7.5-micrometre (0.0075 mm) thick sheet ofpolyimide. The polyimide sheet had a mass of about 10 grams per square metre. A thin-film solar array is embedded in the sail. EightLCD panels are embedded in the sail, whose reflectance can be adjusted forattitude control.[83][84] IKAROS spent six months traveling to Venus, and then began a three-year journey to the far side of the Sun.[85]
A team from the NASAMarshall Space Flight Center (Marshall), along with a team from the NASAAmes Research Center, developed a solar sail mission called NanoSail-D, which was lost in a launch failure aboard aFalcon 1 rocket on 3 August 2008.[86][87] The second backup version,NanoSail-D2, also sometimes called simply NanoSail-D,[88] was launched withFASTSAT on aMinotaur IV on November 19, 2010, becoming NASA's first solar sail deployed in low earth orbit. The objectives of the mission were to test sail deployment technologies, and to gather data about the use of solar sails as a simple, "passive" means of de-orbiting dead satellites and space debris.[89] The NanoSail-D structure was made of aluminium and plastic, with the spacecraft massing less than 10 pounds (4.5 kg). The sail has about 100 square feet (9.3 m2) of light-catching surface. After some initial problems with deployment, the solar sail was deployed and over the course of its 240-day mission reportedly produced a "wealth of data" concerning the use of solar sails as passive deorbit devices.[90]
NASA launched the second NanoSail-D unit stowed inside the FASTSAT satellite on the Minotaur IV on November 19, 2010. The ejection date from the FASTSAT microsatellite was planned for December 6, 2010, but deployment only occurred on January 20, 2011.[91][needs update]
On June 21, 2005, a joint private project betweenPlanetary Society,Cosmos Studios andRussian Academy of Science launched a prototype sailCosmos 1 from a submarine in theBarents Sea, but theVolna rocket failed, and the spacecraft failed to reach orbit. They intended to use the sail to gradually raise the spacecraft to a higher Earth orbit over a mission duration of one month. The launch attempt sparked public interest according to Louis Friedman.[92] Despite the failed launch attempt of Cosmos 1,The Planetary Society received applause for their efforts from the space community and sparked a rekindled interest in solar sail technology.
On Carl Sagan's 75th birthday (November 9, 2009) the Planetary Society announced plans[93] to make three further attempts, dubbedLightSail-1,-2, and -3.[94] The new design will use a 32 m2 Mylar sail, deployed in four triangular segments like NanoSail-D.[94] The launch configuration is a 3UCubeSat format, and as of 2015, it was scheduled as a secondary payload for a 2016 launch on the firstSpaceXFalcon Heavy launch.[95]
"LightSail-1" was launched on 20 May 2015.[96] The purpose of the test was to allow a full checkout of the satellite's systems in advance of LightSail-2. Its deployment orbit was not high enough to escape Earth's atmospheric drag and demonstrate true solar sailing.
"LightSail-2" was launched on 25 June 2019, and deployed into a much higher low Earth orbit. Its solar sails were deployed on 23 July 2019.[97] It reentered the atmosphere on 17 November 2022.
The Near-Earth Asteroid Scout (NEA Scout) was a mission jointly developed byNASA'sMarshall Space Flight Center (MSFC) and theJet Propulsion Laboratory (JPL), consisting of a controllable low-costCubeSat solar sail spacecraft capable of encounteringnear-Earth asteroids (NEA).[98] Four 7 m (23 ft) booms were to deploy, unfurling the 83 m2 (890 sq ft) aluminized polyimide solar sail.[99][100][101] In 2015, NASA announced it had selected NEA Scout to launch as one of several secondary payloads aboardArtemis 1, the first flight of the agency's heavy-liftSLS launch vehicle.[102] However, the craft was considered lost with the failure to establish communications shortly after launch in 2022.[103]
The NASA Advanced Composite Solar Sail System (ACS3)[104] is a technology demonstration of solar sail technology for future small spacecraft.[105] It was selected in 2019 by NASA'sCubeSat Launch Initiative (CSLI) to be launched as part of theELaNa program.[106]
ACS3 consists of a 12U (unit)[107]CubeSat small satellite (23 cm x 23 cm x 34 cm; 16 kg) that unfolds a quadratic 80 square metres (860 sq ft) solar sail consisting of apolyethylene naphthalate film coated on one side withaluminum for reflectivity and on the other side withchromium to increase thermal emissivity. The sail is held by a novel unfolding system of four 7 metres (23 ft) longcarbon fiber reinforced polymerbooms that roll-up for storage.[108]
ACS3 was launched on 23 April 2024 on theElectron "Beginning Of The Swarm" mission. The ACS3 successfully made contact with ground stations following deployment in early May.[109] The solar sail was confirmed as successfully operational by mission operators on 29 August 2024.[110][111]
On 25 October 2024 it was reported "... a bent support arm has made it (ACS3) lose direction and spin out of control in space."[112]
Despite the losses ofCosmos 1 and NanoSail-D (about 23cm x 23cm x 34cm) which were due to failure of their launchers, scientists and engineers around the world remain encouraged and continue to work on solar sails. While most direct applications created so far intend to use the sails as inexpensive modes of cargo transport, some scientists are investigating the possibility of using solar sails as a means of transporting humans. This goal is strongly related to the management of very large (i.e. well above 1 km2) surfaces in space and the sail making advancements. Development of solar sails for crewed space flight is still in its infancy.
A technology demonstration sail craft, dubbedSunjammer, was in development with the intent to prove the viability and value of sailing technology.[113]Sunjammer had a square sail, 38 metres (125 ft) wide on each side, giving it an effective area of 1,200 square metres (13,000 sq ft). It would have traveled from the Sun-EarthL1Lagrangian point 1.5 million kilometres (930,000 miles) from Earth to a distance of 3 million kilometres (1.9 million miles).[114] The demonstration was expected to launch on aFalcon 9 in January 2015.[115] It would have been a secondary payload, released after the placement of theDSCOVR climate satellite at the L1 point.[115] Citing a lack of confidence in the ability of its contractorL'Garde to deliver, the mission was cancelled by NASA in October 2014.[116]
OKEANOS (Outsized Kite-craft for Exploration and Astronautics in the Outer Solar System) was a proposed mission concept by Japan'sJAXA to Jupiter'sTrojan asteroids using a hybrid solar sail for propulsion; the sail would have been covered with thinsolar panels to power anion engine.In-situ analysis of the collected samples would have been performed by either direct contact or using a lander carrying a high-resolution mass spectrometer. A lander and a sample-return to Earth were options under study.[117] The OKEANOS Jupiter Trojan Asteroid Explorer was a finalist for Japan'sISAS' 2nd Large-class mission to be launched in the late 2020s. However, it was not selected.
In August 2019, NASA awarded theSolar Cruiser team $400,000 for nine-month mission concept studies. The spacecraft would have a 1,672 m2 (18,000 sq ft) solar sail and would orbit the Sun in a polar orbit, while thecoronagraph instrument would enable simultaneous measurements of theSun's magnetic field structure and velocity ofcoronal mass ejections.[118] If selected for further development, it would have launched in 2025. However,Solar Cruiser was not approved to advance to phase C of its development cycle and was subsequently discontinued.[119]
As of December 2013[update], theEuropean Space Agency (ESA) has a proposed deorbit sail, named "Gossamer", that would be intended to be used to accelerate the deorbiting of small (less than 700 kilograms (1,500 lb)) artificial satellites fromlow Earth orbits. The launch mass is 2 kilograms (4.4 lb) with a launch volume of only 15×15×25 centimetres (0.49×0.49×0.82 ft). Once deployed, the sail would expand to 5 by 5 metres (16 ft × 16 ft) and would use a combination of solar pressure on the sail and increased atmospheric drag to accelerate satellitereentry.[44]
The well-funded Breakthrough Starshot project announced on April 12, 2016, aims to develop a fleet of 1000 light sail nanocraft carrying miniature cameras, propelled by ground-based lasers and send them toAlpha Centauri at 20% the speed of light.[120][121][122] The trip would take 20 years.
.jpg)
Cordwainer Smith gives a description of solar-sail-powered spaceships in "The Lady Who Sailed The Soul", published first in April 1960.
Jack Vance wrote a short story about a training mission on a solar-sail-powered spaceship in "Sail 25", published in 1961.
Arthur C. Clarke andPoul Anderson (writing as Winston P. Sanders) independently published stories featuring solar sails, both stories titled "Sunjammer," in 1964. Clarke retitled his story "The Wind from the Sun" when it was reprinted, in order to avoid confusion.[123]
InLarry Niven andJerry Pournelle's 1974 novelThe Mote in God's Eye, aliens are discovered when their laser-sail propelled probe enters human space.
A similar technology was the theme in theStar Trek: Deep Space Nine episode "Explorers". In the episode, Lightships are described as an ancient technology used byBajorans to travel beyond their solar system by using light from the Bajoran sun and specially constructed sails to propel them through space ("Explorers".Star Trek: Deep Space Nine. Season 3. Episode 22.).[124]
In the 2002Star Wars filmAttack of the Clones, the main villainCount Dooku was seen using a spacecraft with solar sails.[125]
In the 2009 filmAvatar, the spacecraft which transports the protagonistJake Sully to the Alpha Centauri system, theISV Venture Star, uses solar sails as a means of propulsion to accelerate the vehicle away from the Earth towards Alpha Centauri.
In the third season ofApple TV+'salternate history TV showFor All Mankind, the fictional NASA spaceship Sojourner 1 utilises solar sails for additional propulsion on its way to Mars.
In the final episode of the first season of 2024Netflix TV show,3 Body Problem, one of the protagonists, Will Downing, has hiscryogenically frozen brain launched into space toward the oncoming Trisolarian spaceship, using solar sails andnuclear pulse propulsion to accelerate it to a fraction of the speed of light.
{{cite journal}}: CS1 maint: multiple names: authors list (link)On September 4, the MESSENGER team announced that it would not need to implement a scheduled maneuver to adjust the probe's trajectory. This is the fourth time this year that such a maneuver has been called off. The reason? A recently implemented navigational technique that makes use of solar-radiation pressure (SRP) to guide the probe has been extremely successful at maintaining MESSENGER on a trajectory that will carry it over the cratered surface of Mercury for a second time on October 6.
Sometimes the satellite is called NanoSail-D and sometimes NanoSail-D2. ... Dean: The project is just NanoSail-D. NanoSail-D2 is the serial #2 version.
Planetary Society, ... the next three years, ... series of solar-sail spacecraft dubbed LightSails
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